(US transit agencies are unreasonably ignorant of best practices, including electrification and EMUs, but it's still rail.)
It's also much easier to compare as a baseline of decent rail infrastructure, since it implies a minimum condition of the line and a certain amount of investment in the last 100 years (and it was much easier to compare for the NE corridor, since that contains most electrified rail in the US). Most countries that are considered to have a great rail network have a lot of electrified lines, beginning with Switzerland but countries as Russia have also invested a lot in electrification. Electrification is a lot of effort, and it will take multiple decades to achieve a decent percentage in the US if it were started right now with a lot of political backing.
Many transit agencies in the US, including the one in Boston, are planning electrified rail (as they're aware of the benefits as well) but are unable to construct it right now (and likely the next 10 years) due to funding and ownership issues.
It's not impossible to run decent service over non-electrified rail, but the slower acceleration, near impossibility of high speed as well as the increasingly low availability of DMUs make it harder and, coupled with the higher fuel costs, unattractive.
Properly assessing the state of the routes without using electrification as an easy shortcut was way too much effort for me.
In short: Just because you have a lot of gravel roads everywhere doesn't mean you have a decent road network
Boston's MBTA owns its tracks (generally all the way to the state border), so ownership isn't the issue. Instead, it's been an issue of opposition to electrification. Ex: https://pedestrianobservations.com/2019/07/23/massachusetts-...
I'm not completely up-to-date on this, though -- has it gotten better in the last couple years?
Very glad germany has the opposite problem (Schönrechnen), where expected value is artificially kept high and expected costs low for politically wanted rail projects. It's also bad, but less so?